1. Field of the Invention
The invention relates to a circuit and a method for the control of a stepper motor.
2. Related Technology
Stepper motors are used in units and modules in the fields of automotive or office communication, medical engineering, toolmaking, consumer electronics, building services or measurement technology. They are suitable drives for the transformation of digital information to mechanical movement.
Due to the fast development of the semiconductor industry, particularly because of the greater variety of the transistors, diodes and integrated circuits, stepper motors are used not only in price-sensitive applications, but also where the mechanical commutation system of the dc motors limits the reliability or life of a drive. Stepper motors are more and more used also as controlled auxiliary drives.
Stepper motors have usually a stator winding with several separately switchable stator coils the number of which corresponds to the step angle of the stepper motor. Speed and position of the rotor are controlled by an open control chain. Bipolar driver ICs as well as discrete unipolar transistors are used for the control. Therefore the control unit can always determine the current position of the rotor and, dependent on the result of the determination, move to a new position.
Different operational conditions can be realized dependent on the wiring and control of the coils. On principle, stepper motors may be classified as unipolar and bipolar stepper motors.
In unipolar stepper motors only one coil half of the stator coils is energized. While the central taps of the coils are connected to the supply voltage, the end terminals of the coils each are grounded over an associated switch so that for the unipolar operational mode with two coils, five connection lines to the motor and four switches for control are necessary.
In bipolar stepper motors the coils are energized over their total length. In this case the coils of the stepper motor are operated in an H-bridge circuit each so that the current direction in the magnets can be changed for polarity reversal of the magnetic fields. When two coils are used, four supply lines to the motor and eight switches are necessary.
A plurality of circuit configurations are known in the state-of-the-art.
DE 102 374 34 A1, e.g., discloses a voltage supply for electromotors whereby the output voltage of a dc voltage source with a positive terminal and a negative terminal is halved by means of a voltage phase-balance circuit which in addition to the positive terminal and the negative terminal has a zero potential output that is connected to a terminal of each connected electromotor. At least one further terminal of each connected electromotor is connected to two driver transistors of a half-bridge in each case, which is connected, on the one hand, to the positive terminal and, on the other hand, to the negative terminal.
In DE 199 25 451 A1, an apparatus for driving a stepper motor using a simplified circuit structure and a bipolar driver system is described. It is characteristic of this invention that the stepper motor has two excitation coils driven by the respective bridge circuits. The transistors provided on two sides of a bridge circuit are used by both bridge circuits in common. Therefore the driver circuit can be designed having a reduced number of transistors.
Another driver circuit for a stepper motor is disclosed in WO 01/31773. The driver circuit consists of one or several pairs of excitation coils, whereby each excitation coil has two end contacts and a central tap with the central taps of the excitation coils connected to each other. Moreover, switching elements are provided that are designed such that the positive terminal of the supply voltage is energized over an end contact of a first excitation coil and the negative terminal of the supply voltage is energized over a second excitation coil.
In U.S. Pat. No. 4,558,268 a circuit for the control of stepper motors is described that is provided with several half-bridges for two stepper motors. In this way the number of the needed switching elements can be reduced, but the stepper motors, however, cannot be operated independently of each other.
In EP 0 657 990 A1, however, a method for the simultaneous control of several stepper motors is disclosed that uses a diode multiplex. Also, here switching elements can be saved, but additional diodes are required at the stepper motors. Moreover, also this method does not allow to operate the stepper motors independently of each other.
It is the object of the invention to propose a circuit and a method for the control of a stepper motor that require only a small number of switching elements and connection lines and, in addition, ensure to operate serially connected stepper motors independently of each other.